The latest engine development trends have blended the consumers demands for high performance and horsepower output with Governmental regulations for fuel efficiency and emission reductions. As a result of this, it has become increasingly common to use a four valve per cylinder design. One application in particular is a four-stroke cycle engine for motorcycles. In order to incorporate all of these elements into one design, space constraints being considered, spark plugs of a reduced size were developed. To this end, compact spark plugs attain the following advantages over the conventionally sized plugs:
Improved volumetric efficiency due to an increase in valve diameters and a reduction in valve arrangement angles.
Improved rigidity of the material in the area between the threaded plug hole and the valve seat.